The present invention relates to a visual inspection method and apparatus which perform detection and classification of defect, such as a fine pattern defect and a foreign particle, based on an image of an object obtained using a light of a lamp, a light of a laser, or an electron beam, for thin film devices such as a semiconductor wafer, TFT, and a photomask. The present invention relates especially to a visual inspection method and apparatus which are suitable to conduct the visual inspection of a semiconductor wafer.
Thin film devices, such as a semiconductor wafer, a liquid crystal display, and a magnetic head for a hard disk drive, are manufactured through many fabrication steps. In manufacture of such thin film devices, a visual inspection is performed for every series of several fabrication steps for the purpose of the improvement and stabilization in the yield. In the visual inspection, a pattern defect or a foreign particle-caused defect is detected, based on a reference image and an inspection image which are obtained by illuminating corresponding regions in two patterns originally formed to be of identical shape, by a light of a lamp, a light of a laser, or an electron beam. That is, after making registration of the reference image and the inspection image, a difference is calculated and compared with a threshold defined separately. Subsequently, a region where the difference is larger than the threshold is detected as a defect or a foreign particle. As one of threshold calculating methods, JP-A-2000-105203 discloses a method of calculating a threshold as a fixed multiple of a standard deviation indicative of the degree of the variation, corresponding to the variation in the detection signal of a region which is originally formed to be of identical shape, or its neighborhood.
In such an inspection, in order to detect a minute defect, it is necessary to make judgment by setting the threshold low. However, the threshold which is set too low will bring about many false alarms due to the minute difference of a pattern arising from a sampling error, roughness, or grains, or due to uneven brightness caused by film thickness unevenness. From the view point of the purpose of the inspection, a false alarm is completely unnecessary. Therefore, the threshold is often set high to such an extent that the ratio of a false alarm becomes small enough in the entire wafer, thereby performing the inspection at the sacrifice of sensitivity in many cases. For improving sensitivity, JP-A-2004-79593 discloses a method in which an entire wafer or a die is divided into regions, and a defect judgment is performed using different thresholds for every region. According to this technique, it is possible to suppress generation of false alarms, without reducing sensitivity in the entire wafer.
JP-A-2006-266872 discloses a visual inspection method and apparatus which are operable to detect a defect based on an inspection image by a compare check, operable to calculate feature of the defect at the same time, and perform defect classification according to the classification condition set up previously to classification condition setting means. The classification condition setting means collects the feature of the defects covering a large number of defects previously obtained by defect detecting means, samples a defect based on a defect feature distribution, and sets up a defect classification condition based on it.